The present invention relates to an aluminum electrolytic capacitor.
Conventional aluminum electrolytic capacitor, as depicted in FIG. 4, comprises a capacitor element 1, wherein an anode foil 4 having a chemical formed coating on an etching foil made of aluminum and a cathode foil 3 made of an etched aluminum are wound through a separator 2 such as an insulating paper. An anode internal terminal 6 and a cathode internal terminal 5, both having an elongated shape, are connected to said anode and cathode foils 4 and 3, respectively, by way of stitch and cold welding etc. Said capacitor element 1 is impregnated with an electrolyte, and is housed in an outer casing having a tubular shape with a bottom cover. An opening of the outer casing is encapsulated with a sealing member, and the positive/negative internal terminals are connected to the positive/negative external terminals, respectively, each terminal extending from the capacitor element by means of a rivet disposed on said sealing member, whereby an electrolytic capacitor is formed.
When a conventional capacitor is used in a charge/discharge circuit, higher voltage is applied to said negative internal terminal 5 than to the negative foil 3, because the surface area of the negative internal terminal 5 is smaller than that of the negative foil 3. On account of this, there is a possibility that at the time of discharge, such products as hydrogen gas may be generated at said negative internal terminal 5 and the negative foil 3 around the terminal 5 as the film generation reaction develops; and that this generation of hydrogen gas may bring about higher internal pressure of the capacitor, causing such a trouble as activation of an explosion-proof valve.
A conventional invention directed to improvement the aforementioned problem has a configuration in which a surface, facing a positive separator, of a negative internal terminal 5 connected to a negative foil 3 is roughened and in which a surface of the terminal 5, facing the negative foil 3, is flattened.
Patent Document 1: Japanese Laid-open Patent Publication No. 2005-39132
Today, however, an aluminum electrolytic capacitor for use in a servomotor etc. has a shorter charge/discharge period and such voltages are applied that charge/discharge voltage difference is greater. Another problem has occurred in such a use, e.g. short circuit, as well as activation of safety valve.
The present invention is proposed for solving the above-mentioned problems relating to conventional electrolytic capacitors, and is directed to achieve longer durability of an electrolytic capacitor for use in a charge/discharge circuit with shorter period and greater voltage difference.
As part of extensive research for solving the above-mentioned problems, the present inventors carried out an analysis of an electrolytic capacitor having a short circuit in order to figure out the cause of the short circuit that occurs during usage of the electrolytic capacitor carrying a charge/discharge circuit, and found that an iron portion was exposed on the surface of the negative internal terminal. This phenomenon can be identified in terms of electrochemical mechanism by the fact that at the time of charging, the electrolyte around a cathode foil and a cathode internal terminal is alkalized by means of the reduction reaction of oxygen and water and the reduction reaction of hydrogen, where, under a condition that a lot of charge current flows such as usage of an electrolytic capacitor installed in a charge/discharge circuit with great voltage difference, the above-stated reduction reaction frequently occurs, which significantly affects the alkalization of the electrolyte around the cathode foil and the cathode internal terminal. The reduction reaction occurs on the entire surface of the cathode internal terminal if the content of dissimilar metal contained in the cathode internal terminal is evenly distributed on the entire area thereof; whereas the reduction reaction occurs intensively in the interface between the dissimilar metal and the electrolyte if there are some areas of the cathode internal terminal intensively containing dissimilar metal. In a conventional cathode internal terminal, a less fine aluminum material is usually employed because the terminal is too soft to have a stable connection to the cathode foil, and too expensive to produce, in comparison with a fine aluminum material (with degree of purity being 99.99% or greater, for example). Accordingly, because the content of dissimilar metal, in particular, iron metal, in a cathode internal terminal is greater than that in a cathode foil, a reduction reaction of iron contained in the cathode internal terminal and the electrolyte at the interface between the terminal and the foil frequently occurs, leading to alkalization of the electrolyte around the cathode internal terminal. This causes the oxide film and aluminum on the surface of the cathode internal terminal to dissolve, leading to exposure of iron which is a dissimilar metal contained in the cathode internal terminal, whereby the current intensifies on the iron, which generates a short circuit.
It has been discovered by the applicants of the present invention that a short circuit can be prevented by using these parts with the dissimilar metal adjacent to the surface of the cathode internal terminal being significantly reduced in amount, finally leading to the achievement of the present invention.
It has also been discovered that a great amount of dissimilar metal exists not only in the cathode internal terminal but also in the volute tongue of the capacitor element and in the proximity of the cathode internal terminal. Besides, it has been found that when the anode foil firstly starts being wound and then the cathode foil is wound together, a short circuit occurs at a portion of the anode foil facing the end of the cathode foil and at a portion of the anode foil facing the cathode internal terminal. It has further been discovered that a short circuit occurs after the charge/discharge voltage is applied when the charging is finished. The configuration of an electrolytic capacitor is now considered. An oxide film of an anode foil functions as a dielectric material, and an electric charge is accumulated between the oxide film and the cathode foil facing therewith, thus forming a capacitor. If a portion of the anode foil does not face with any portion of the cathode foil, or if the portion of the anode foil faces with only a small area of the cathode foil, the electric charge on the anode foil concentrates either on the portion thereof closest to such portions of the cathode foil or on the portion thereof facing with the small area of the cathode foil. In addition, it has been supposed that the chemical formed coating is degraded by means of a charge/discharge application under unprecedentedly severe conditions, and that a great voltage is further applied to the chemical formed coating with the electric charge being maximally accumulated after the charging is finished, whereby a short circuit is generated. To be more specific, when the anode foil firstly starts being wound and then the cathode foil is wound together, the end portion of the anode foil does not face with any portion of cathode foil; wherein all the electric charge on the end of the anode foil not facing with any portion of the cathode foil concentrates on the portion of the anode foil facing the end of the cathode foil, leading to a short circuit. On the other hand, a small and flat cathode internal terminal faces with a portion of the anode foil, on which the electric charge concentrates, leading to a short circuit. The discussion rendered above is a consideration of an electrolytic capacitor using an electrolyte designed for driving an electrolytic capacitor; however, a similar behavior can be conceived with respect to an electrolytic capacitor using a solid electrolyte.